They first started to claim working families tax credit (as it was then called) back in December 2002.

Money started to come in from the summer of 2003.

Then in February 2004, for the first time, a formal award notice arrived, telling them they were entitled to nearly £10,000 a year for 2003-04.

Suspicious about the size of the award, they also noticed two apparent errors.

The first was that the award overstated Guy's income. And secondly it suggested that Sanna was in receipt of job seeker's allowance.

"I had never claimed I was receiving job seeker's allowance," says Sanna.

After contacting the tax credit office several times they were told the money was theirs, so they spent it on nursery fees for their child.

But the consequences of making a stray mark on just one page of that original claim form are still reverberating more than four years on.

The nightmare begins

In February 2005, out of the blue, came a demand for the repayment of £6,500.

An accidental mark across the corner of the box has triggered the wrong conclusion. Nobody would count that as a vote in an election

Simon Hughes MP

Then last year, the tax credit office demanded £1,500 more, for 2004-05.

Requests for an explanation of what had gone wrong were largely ignored.

However in one phone conversation, a civil servant did suggest that the problem might have been due to the original claim form from three years earlier.

Crossing out part of the original form as "not applicable", with the pen straying across the corner of one box, had apparently led either a person or a computer to decide that Sanna was claiming to be in receipt of job seeker's allowance.

In turn, this overrode all the other information on the application, and led to payments in 2003-04 and 2004-05 that were much larger than would otherwise have been the case.

Since then, the couple's tax credit award has fallen back to just £10 a week.

An MP steps in

Their MP - Simon Hughes, the Liberal Democrat MP for North Southwark & Bermondsey - has taken up their case, writing to the tax credit office three times and asking for a meeting with someone in authority.

"Unusually and bizarrely, an accidental mark across the corner of the box has triggered the wrong conclusion. Nobody would count that as a vote in an election," Mr Hughes says.

"Surprisingly they (the tax credit office) haven't accepted that what the couple says is correct," he adds.

But over the past six or seven months, even he has not been able to get a coherent explanation for his constituents' problem, nor been able to set up a meeting to thrash things out.

"We would love them to have the courage and the competence to have a meeting to go through the documents," says Guy.

"They haven't responded to the idea in any way whatsoever."

Some rigid bureaucracy

Dealing with the tax credit office over the past few years has been just as stressful for Sanna and Guy as the demand for the repayment of money.

Five years worth of tax credit documents built up by Sanna and her husband

Despite numerous requests for an explanation of what has happened, and why they should pay up, none has been forthcoming.

"They have never, to this day, ever condescended to write to us or telephone us to explain why this error happened," says Guy.

Phone calls to correct the first, incorrect, award notice were made repeatedly in 2004, but despite assurances that a new award would be made, in fact the same errors were simply repeated.

Even when they wrote to the tax credit office in 2004, with full supporting documentation, telling the civil servants they were still making the same errors - overestimating income and assuming that Sanna was getting job seeker's allowance - they were given the brush-off.

"That letter was returned by return of post, saying 'please do not write to us again unless we ask you to'," says Guy.

"That was the only letter they have ever written to us."

Meanwhile the tax credit office has denied the existence of some of the dozens of phone calls from the couple over the past four years, even accusing them at one point of faking their itemised telephone bills.

No response

The couple have lodged two formal appeals against the demand for repayment.

We are convinced we have acted correctly and that HMRC did not make a mistake when we processed their claim

HMRC

Last year Sanna and Guy turned to Citizens Advice for help, where they gained an interesting insight into tax credit problems generally.

"They said, 'We see 3 or 4 cases a day like this - you will never get any explanation, you will never get a reply, you will never get a letter'," said Sanna.

"We advise you to tell them to take you to court, call their bluff, because they have no idea what they are doing," she was told.

Another long letter has been sent, again outlining the saga in full, supplying supporting documentation once more, and asking why they should pay for HMRC's mistake.

Movement?

Till now, there had been no response.

However, following the BBC's enquiries, HMRC has suggested a meeting with Mr Hughes and his constituents at their local tax office.

The authorities have confirmed that the pen mark on the original application form lies at the heart of the matter.

But they are adamant that Sanna Wennberg was at fault for filling in the form incorrectly.

"We have looked at this case very closely," said a spokeswoman.

"We are convinced we have acted correctly and that HMRC did not make a mistake when we processed their claim. That means the overpayment must be repaid," she said.

HMRC also says that it sent an initial award notice to the couple as early as 2003 - something the couple deny having received - and that this should have revealed the errors.

"We have written to them with details on how they can arrange to pay," said the HMRC spokeswoman.

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